Mechanical Genesis of the Scales of Fishes. 247 



It will be clear that in the case considered the arrangement 

 and imbiication of the scales is determined by tlie actions of 

 the seg-mentally arranfjefl muscles of the body. In other words, 

 whatever lias determined the development of somites has also, 

 in the most clear and direct manner, determined the segmen- 

 tally recurrent and peculiar trilinear and im'bricated arrange- 

 ment of the scales of many fishes. It may be urged in 

 objection that heredity has determined the number, arrange- 

 ment, and the development of the somites, and therefore the 

 development of the scales is also a sequence of hereditary 

 influences working thus indirectly. This view of the case may 

 be admitted without invalidating the conclusion that, given 

 the growing mechanism here described, tlie development of 

 the scales would under any circumstances have been interfered 

 Avith at the jioints where the integument was being continually 

 Hexed, wrinkled, or folded as it is around the iutegumentury 

 areola? wherein the scales are formed, as has been here proved 

 to correspond with the facts. 



Special types of squamation amongst fishes may require an 

 interpretation different as to details from the foregoing; but 

 it is probable that such special cases will rather tend to con- 

 firm than otherwise the views develo|)ed in this sketch of an 

 hypothesis respecting the mechanical origin of the arrange- 

 ment and imbrication of the scales of fishes. For example, 

 one of the most extreme cases, that of the sturgeon, shows 

 that the smaller integumentary plates between the large 

 doisal, lateral, and ventral rows conform to these lines of 

 tension of tlie myocommata upon the integument. An even 

 more instructive example is that of the common eel, in which 

 the scales are oblong rhombs or parallelograms, arranged with 

 their diameters in oblique lines, running in two directions 

 conformably with the tensions, wrinklings, and foldings of 

 the integuments produced by the oblique insertions of the 

 muscles when the latter are brought into action. Other cases 

 where the scales are very fine might be urged in objection, 

 especially where several oblique rows of scales are found to 

 correspond to each somite. Such parallel duplication of scale- 

 rows, however, does not invalidate the principle, since the rows 

 still conform to the lines of tension of the linear attachment 

 of the myocommata to the integuments. The hypothesis may 

 also be extended so as to consistently consider such forms as 

 the pipe-fishes and other anomalous forms, where sluggish 

 habits coupled with the almost exclusive use of the dorsal fin 

 in swimming has rendered the lateral musculature of the 

 body comparatively subordinate in function, and which may 

 even lead to secondary fusion of somites and the consolidation 

 of consecutive pairs or triplets of vertebral centra into single 

 vertebral bodies. 



18* 



